1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process, such as a printer, a copying apparatus or a facsimile apparatus, and more particularly to a drive of an image bearing member or a driven member acting on the image bearing member.
2. Description of the Related Art
As an image forming apparatus utilizing for example an electrophotographic process, there is known a color image forming apparatus of so-called in-line type including a plurality of image forming parts, each including an electrophotographic photosensitive member (hereinafter referred to as “photosensitive member”) constituting an image bearing member, and process means which acts on such photosensitive member. In the image forming apparatus of such in-line type, images on the photosensitive member are transferred in succession onto an intermediate transferring member disposed in opposition to the plurality of image forming parts or onto a transfer material on a transfer material bearing member. In general, the plurality of image forming parts are constituted of four image forming parts for respectively forming images of yellow, magenta, cyan and black colors, and a drum-shaped member is used as the photosensitive member (4-drum tandem type).
Also in the image forming apparatus of such in-line type, it is known to integrally construct the photosensitive member and the process means such as developing means, in each image forming part, as a process cartridge, that is rendered detachably mountable in a main body of the image forming apparatus. In such process cartridge system, for example when a developer is used up, the user himself can replace the process cartridge without relying on a service personnel to enable image formation again. Also other consumable parts such as the photosensitive member may also be replaced at the same time, whereby the maintenance ability can be significantly improved.
As an example, an image forming apparatus of the aforementioned 4-drum tandem system will be explained below. Currently, a “4-motor system” as shown in FIG. 18 is commonly employed, in which four photosensitive members 1a, 1b, 1c and 1d are driven by respective motors 101a, 101b, 101c and 101d. 
Also a “1-motor system” as shown in FIG. 19 is employed, in which all four photosensitive members 1a, 1b, 1c and 1d are driven by a single motor 101.
In the “1-motor system”, an electronic clutch or a mechanical clutch mechanism is utilized for switching over a full-color mode and a mono-color mode. The full-color mode is a multicolor mode in which all four photosensitive members 1a, 1b, 1c and 1d are driven, while the mono-color mode is a monochromatic mode in which for example a photosensitive member 1d for black color only is driven. The “1-motor system” has an advantage in cost, in comparison with the “4-motor system”.
The “4-motor system” tends to become costly, because of an increased number of motors. Also the “4-motor system” may lead to an increased installation space and an increased weight in the structure, and may hinder realizing a compact and low-cost image forming apparatus.
On the other hand, the “1-motor system” may cause an unevenness in the rotation of the photosensitive member, depending on the precision of meshing in the clutch mechanism, thereby conspicuously causing a color misalignment or a banding phenomenon (periodical unevenness in image by a fluctuation in the rotation of the photosensitive member).
Japanese Patent Application Laid-open No. 2003-43781 discloses a technology of driving photosensitive drums for yellow, magenta and cyan colors with a first motor, and driving a photosensitive drum for black color with a second motor.
However, in the technology of Japanese Patent Application Laid-open No. 2003-43781, the first motor also drives developing rollers corresponding to the photosensitive drums driven by the first motor, and the second motor also drives a developing roller corresponding to the photosensitive drum driven by the second motor, and does not pay attention to a driving load relating to the photosensitive drum and the developing roller.